Generally, an airbag is an apparatus for protecting a driver and passengers during a head-on collision of a vehicle driving at a speed of about 40 km/h or more, by sensing collision impact of the vehicle with an impact sensor, and then exploding gunpowder to supply a gas into the airbag, thereby inflating it.
Characteristics required of the fabric for an airbag are low air permeability for unfolding the airbag well at the time of collision, high strength and high heat resistance for preventing damage to and rupture of the airbag itself, and flexibility for reducing the impact provided to occupants.
Particularly, the airbag for a vehicle is prepared in a certain shape and installed in a steering wheel, side windows, or side structures of the vehicle in a folded form to minimize its volume, and it is expanded and unfolded when an inflator operates.
To ensure superior expanding and unfolding performances upon unfolding of the airbag by abrupt gas generation in the inflator, air-tightness of the airbag cushion may be increased by maintaining the correct shape in the warp or weft direction. However, a polyamide fiber such as nylon 66 which has been previously used in the preparation of an airbag cushion is generally sensitive to temperature and speed, and thus it is difficult to maintain the correct shape in the warp or weft direction upon cutting a fabric. Particularly, in the case of a large-sized cushion, fabric cutting is not performed accurately, which generates problems of poor appearance and reduced productivity.
Meanwhile, Japanese Patent Publication No. Heisei 04-214437 suggested use of a polyester fiber in a fabric for an airbag in order to reduce the drawbacks of the polyamide fiber. However, when the airbag is manufactured by using the prior polyester fiber, it is difficult to install the airbag in a narrow space in a vehicle because of its high stiffness, excessive thermal shrinkage is caused by heat-treatment at a high temperature due to its high modulus and low elongation, and there is a limitation in maintaining sufficient mechanical and unfolding properties under severe conditions of high temperature and high humidity.
Further, when a polyester yarn is applied to weave a high-density fabric for an airbag, a force applied to a weft insertion region is not the same as a force applied to a region opposite to the weft insertion region, and therefore a force applied to a yarn in the weft insertion region becomes higher than a force applied to a yarn in the region opposite to the weft insertion region, such that a fabric in the region opposite to the weft insertion region is not woven firmly, resulting in wrinkle generation in a selvage of the fabric.
Due to this problem, a coating agent is not evenly applied onto the whole fabric upon processing and coating, and thermal stress remaining in the fabric used in a vehicle airbag is released, and as a result, shrinkage of the fabric occurs. Further, this shrinkage deformation characteristic may cause a change in the intrinsic weaving density of the fabric to generate problems such as reductions in air permeability performance and dimensional stability, changes in the volume and thickness of a final cushion product, etc.
Accordingly, there is a need to develop a process capable of effectively preparing a polyester fabric for an airbag, which enables the whole fabric to be provided with uniform tension upon weaving a high-density fabric for an airbag using a polyester yarn, and is able to effectively prepare a polyester fabric for an airbag having superior mechanical properties and air-tightening effect to be suitable as a fabric for a vehicle airbag.